The present invention relates to polymeric polyols prepared by the polymerization of epoxy resins.
Epoxy resins are commercially important materials that are used extensively to make thermosetting products for use in coatings, adhesives, composites, and many other applications. The largest volume of epoxy resins utilized in commerce are those based upon the diglycidyl ether of bisphenol-F (DGEBF), epoxy novolac resins, and those based upon the diglycidyl ether of bisphenol-A (DGEBA). Of these, the bisphenol-A based products are utilized in much larger volumes than the other products.
Bisphenol-A derived epoxy resins are essentially linear polymers available in a wide range of molecular weights, represented generically by the following chemical structure: 
where n represents the average number of repeat units in the polymer. The low end of the range of available molecular weight products are made by reaction of bisphenol-A with excess epichlorohydrin, followed by treatment with base. They are referred to by those who work in the industry as liquid epoxy resin, BADGE, or DGEBA, even though most of the commercial products are not pure DGEBA but often have a value of n of about 0.15 or slightly higher. Higher molecular weight epoxy resins (greater than about 400 Daltons) are commercially prepared by the so-called xe2x80x9cadvancement processxe2x80x9d which is the reaction of excess DGEBA with bisphenol-A, where the ratio of DGEBA to bisphenol-A is used to control the final average molecular weight.
Epoxy resins contain epoxide rings at the chain ends, and (with the exception of pure DGEBA) secondary hydroxyl groups spaced along the polymer backbone. Both of these functional groups can be utilized to cure the epoxy resin. For example, multifunctional amines, mercaptans, and carboxylic acids are utilized to crosslink through the epoxide ring. Amino resins such as melamine-formaldehyde and urea-formaldehyde resins, and polyisocyanates are utilized to crosslink through the hydroxyl groups. Finally, resins such as resoles crosslink through both the hydroxyl and epoxide functional groups. For most purposes, epoxy resins that are crosslinked via the epoxide end groups have epoxy equivalent weights (EEW) of at most about 800, and frequently far less than this. On the other hand, when crosslinked through hydroxyl groups, higher molecular weight epoxy resins are generally preferred, and very low molecular weight epoxy resins such as pure DGEBA which lack OH groups cannot be utilized at all in such a thermosetting system.
Because high molecular weight epoxy resins are prepared by the reaction of DGEBA and bisphenol-A, such resins prepared using current commercial processes have relatively high levels of residual bisphenol-A and DGEBA in the final products. Unfortunately, these compounds are of concern with regard to their human health effects and pseudo-estrogenic activity. This is particularly true in the industry for coatings for food and beverage can interiors, where epoxy resins are currently utilized in large volumes for coatings that are crosslinked with amino resins and other OH-reactive crosslinking agents. Thus, there is a strong need to develop coatings with properties similar to those obtained from crosslinked epoxy resins, but without such high levels of residual DGEBA and bisphenol-A, which can be extracted into the contents of the can and thus become a component of the human diet.
Despite the fact that epoxy resins can be crosslinked with amino resins and the like through the secondary hydroxyl groups on the resin backbone, it is generally found that significantly higher temperatures and/or bake times are required than are necessary with other polyols utilized in coatings, such as acrylic polyols and polyester polyols. It is thought that the relatively hindered environment of the OH groups on the epoxy resin is responsible for this effect. Obviously, this is usually a significant drawback to the utilization of epoxy resins in such coatings, since higher oven temperatures and/or bake times lead to higher production costs.
The cationic or acid-catalyzed polymerization (or homopolymerization) of multifunctional epoxy resins to yield gelled or crosslinked final products is a well-known process of significant commercial importance. Lewis acids are most commonly employed, but appropriate Brxc3x8nsted acids can also be utilized. For example, C. A. May (Ed.), Epoxy Resins Chemistry and Technology, Marcel Dekker, Inc.: New York, 1988, reports (p. 495) that Lidarik et. al. (Polymer Sci. USSR, 1984, 5, 589) polymerized glycidyl ethers with complexes of antimony pentachloride, boron trifluoride, and perchloric acid. Additional examples are reported in May. In addition, the photoinitiated cationic polymerization of epoxy resins is well-known, and also of commercial importance. As reviewed in May (pp. 496-498), cationic photoinitiators are materials that upon photolysis generate strong Brxc3x8nsted acids, which serve as the true catalyst for the epoxide polymerization.
The copolymerization of water with monofunctional epoxide compounds has been known for some time. For example, R. W. Lenz, Organic Chemistry of Synthetic High Polymers, Interscience Publishers: New York, 1967, pp. 531-546, reviews the ring-opening polymerization of cyclic ethers including epoxides, and notes that C. Matignon, et.al. (Bull. Soc. Chim., 1, 1308 (1934)) studied the effect of water content on the oligomer distributions obtained from the acid-catalyzed hydration of ethylene oxide.
U.S. Pat. No. 6,331,583 B1 discloses compositions of emulsified polymeric polyols prepared by a method comprising the acid catalyzed, non-reversible polymerization of lower molecular weight epoxy resins in an aqueous emulsified state. Coating compositions are prepared from the emulsified polymeric polyols crosslinked with various crosslinking agents.
U.S. Pat. No. 2,872,427 discloses oil-in-water emulsions of polyepoxide resins and their heat cure with various curing agents, including acid acting curing agents.
Multifunctional epoxide compounds and water are copolymerized by treatment with certain acid catalysts, optionally in the presence of a solvent, to produce higher molecular weight polyol products. The molecular weight (Mw or Mn) of the polyol products can be changed by varying the ratio of water to multifunctional epoxide compound.
An embodiment of the invention provides a method which comprises copolymerizing a multifunctional epoxide resin and water in the presence of an effective amount of acid catalyst and optionally in the presence of a solvent that substantially dissolves both the epoxide resin and water, the amount of water being sufficient to avoid gelation. The copolymerization although conducted with water is not conducted in the form of an aqueous dispersion, or emulsion, polymerization leading to an aqueous polymer dispersion, i.e., it comprises a non-dispersion, non-emulsion, copolymerization.
Another embodiment of the invention provides compositions comprising higher molecular weight polyols, or polymeric polyols, having a number average molecular weight (Mn) of at least about two times the molecular weight of the multifunctional epoxy resin from which they are prepared. In the case of DGEBA resin, the Mn would be at least about 750. The polymeric polyols will comprise glycol end groups and a repeat unit structure which contains two glycidyl units, primary and/or secondary alcohols. When prepared according to the inventive copolymerization method, these polyol compositions will also be substantially free of surfactants, i.e., free of emulsifying agents, that are required for aqueous emulsion, or dispersion, polymerization.
Another embodiment of the invention provides compositions comprising higher molecular weight polyols, or polymeric polyols, prepared from diglycidyl ether of dihydric phenols or diols, such as bisphenol-A, in which the level of residual dihydric phenol or diol in the polyol is less than 20 ppm, preferably less than 10 ppm, and the level of residual diglycidyl ether of dihydric phenol or diol is less than 500 ppm, preferably less than 100 ppm.
As yet another embodiment, the higher molecular weight polymeric polyols can be formulated with suitable OH-reactive crosslinking agents, including amino resins such as melamine formaldehyde resins or polyisocyanates, to yield crosslinked films exhibiting excellent properties such as high hardness and solvent resistance at relatively low bake temperatures.